A field trial was conducted to investigate the changes of endogenous hormone concentration (EHC) during formation and thickening of tuberous roots in sweet potato (Ipomoea batatas L.) of different genotypes: Xushu 18, Minamiyutaka, and I. trifida-K123. The results of this study suggested that the concentrations of zeatin riboside (ZR), dihydro-zeatin riboside (DHZR), abscisic acid (ABA), indole-3-acetic acid (IAA), and isopentenyl-adenine (IPA) of Xushu 18 and Minamiyutaka were significantly higher than those of I. trifida-K123. Dry tuberous root yields were positively correlated with the concentration of ABA, ZR, and DHZR at 1 or 5% significant level, but were not obviously correlated with the concentrations of IAA, IPA, and GA4. The concentrations of ABA, ZR and DHZR played very important roles in the tuberous root formation and thickening in sweet potato. The concentrations of ZR, DHZR, ABA, and IAA in the upper or inner portion of tuberous roots were significantly higher than those toward the end of the root or its outer portion. Meanwhile, EHC of large-sized tuberous roots were significantly higher than those of medium- or small-sized tubers during the early thickening period. In the rapid thickening period of tuberous roots, EHC of medium-sized tubers were the highest. However, at the late-thickening period, EHC of small-sized tubers ranked the highest.
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